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ITTC Recommended

Procedures and Guidelines

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Model Manufacture

Ship Models

Effective Date

2002

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Updated by Approved

Specialist Committee: Procedures for Resis-tance, Propulsion and Propeller Open Water

Tests of 23rd ITTC 2002

23rd ITTC 2002

Date Date 2002

Table of Contents

1 PURPOSE OF PROCEDURE2

2 PARAMETERS...2

2.1 Definition of Variables.......................2

3 DESCRIPTION OF PROCEDURE...23.1 Model Manufacture ...........................2

3.1.1 Hull Model ...................................2

3.1.2 Propeller Model............................3

3.1.3 Appendages..................................4

3.2 Turbulence Stimulation.....................4

3.2.1 Hull ..............................................4

3.2.2 Appendages..................................5

3.2.3 Propeller.......................................5

3.3 Preparation for Model Testing .........5

3.3.1 Ballasting and trimming (for

resistance and self propulsion tests) 5

3.3.2 Wax models .....................................6

3.4 Documentation ...................................6

4 REFERENCES6


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Model Manufacture

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Ship Models

1 PURPOSE OF PROCEDUREThe purpose of the procedure is to ensure

the correct manufacture of hull and propeller

models for Resistance, Propulsion and Propel-

ler Open Water Tests.

The tolerances of propeller models for ma-

noeuvring, sea keeping and ice tests can gener-

ally be larger (typically 1.5 to 2 times) than

those used for propulsion or open water tests

whilst those for cavitation tests may be smaller.

PARAMETERS

1.1 Definition of VariablesX, Y, Z Coordinate directions

LPP Length between perpendiculars

(X) (m)

LWL Length on waterline (X) (m)

B Breadth (Y) (m)

T Draught (Z) (m)

Hull Displacement Volume (m3)

Hull Displacement Mass (kg)

AP After Perpendicular

FP Forward perpendicular

AE Propeller expanded blade area(m2)

A0 Propeller disk area (m2)

D Propeller diameter (m)

P Propeller pitch (m)

p PitchDiameter ratio (P/D)

aE Expanded blade area ratio

(AE/Ao)

t Propeller section thickness (m)

c Propeller chord or blade width

(m)

Solidity Factor Ratio of assumed 2-Drotor blade area / disk area

2 DESCRIPTION OF PROCEDURE2.1 Model Manufacture2.1.1 Hull Model

General: The basic requirement is that themodel should be geometrically similar to the

ship wherever it is in contact with the water.

This may not be possible in all cases owing to

different systems and materials of construction

on model scale and full size and it is desirable

that any departures from similarity should be

known and documented.

Materials and construction: Materials used

for ship hull models include wax, wood, high

density closed cell foam and fibre reinforced

plastic (FRP). Models are normally cut from a

lines plan re-drawn from the ship plan. The ap-

plication of CAD/CAM processes allow the

numerical representation of the hull from the

drawing to the cut model. After cutting, the

model is finished by hand. In the case of FRP

construction, a foam or wooden plug of the

model will be manufactured in a similar man-

ner.

Surface finish: The model surface should be

smooth and equivalent to that achieved with a

300 to 400 grit wet and dry paper.

Stations and waterlines: The spacing and

numbering of displacement

stations and waterlines should be properly de-

fined. Displacementsections may be identified

as follows:

- A ten section system numbering from aftwith station 0 at the AP. The number ofstations can also be 21 (20 equally sized


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intervals). The stations are counted from

aft.

- Decimal fraction stations may be intro-duced at the ends as required (such as 9.5,

9.6, 9.7 etc.).

- Stations aft of the AP to be numberednegatively.

- Stations forward of the FP to be numberedpositively in natural succession (10.1, 10.2

etc.).

It should be noted that there are other

methods of numbering the displacement sta-

tions which may be equally acceptable.

Waterlines are identified as follows:

- Waterlines should be spaced as requiredand identified by their height above the

baseline.

- The baseline should be defined as the top-side of keel. In the case of a raked keel the

baseline is parallel to the design waterline

and midway between the height above

base at the AP and FP stations.

Moulded dimensions: Ship lines are nor-

mally drawn to moulded dimensions and model

hulls should also be constructed to moulded

dimensions.

Manufacturing tolerances: Model hull tol-erances for breadth (Y) and draught (depth) (Z)

should be within 1.0 mm and for length

should be within 0.05% length or 1.0 mm

whichever is the larger.

Openings: Openings in the hull should be

manufactured to within 1.0 mm. Lateral

thrust units should be modelled using an ap-

propriate model of the thruster or a two-

dimensional representation using an appropri-

ate solidity factor.

Stability over time: The dimensions of the

hull model should not move outside the rec-

ommended manufacturing tolerances. It should

be noted that the dimensions of wax models

can change appreciable with changes in tem-

perature. For example, a change of 50C may

alter the length of a model by about 0.15% (10

mm for a 7 m model). The model documenta-

tion should include any changes in dimensions

which may have arisen from this source.

2.1.2 Propeller ModelManufacturing tolerances (for self propul-

sion and open water tests):

Propellers having diameter (D) typically

from 150 mm to 300 mm should be finished to

the following tolerances:

Diameter (D)

0.10 mmThickness (t) 0.10 mm

Blade width (c) 0.20 mm

Mean pitch at each radius (P/D): 0.5% of

design value.

Special attention should be paid to the shap-

ing accuracy near the leading and trailing edges

of the blade section and to the thickness distri-

butions. The propeller will normally be com-

pleted to a polished finish. The use of

CAD/CAM processes further enhance the facil-ity to achieve such tolerances.

The manufacture of model ducts, vane

wheels and pre and post swirl vanes should fol-

low the tolerances recommended above for

model propellers used in self propulsion and

open water tests.

Gaps: The gap between the aft side of the

model bossing and the fore side of the propeller

boss should be the minimum required to allowsufficient forward movement of the propeller


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when calibrating the thrust dynamometer but

should not exceed 2 mm with the propeller in

its design position. Rope guards across the gap

should not be produced at model scale.

Propeller / hull clearances: Propeller/hull

aperture clearances should have tolerances

within 1.0 mm and a maximum axial propel-

ler movement of 1.5 mm. These tolerances

should provide propeller clearances with lessthan 1.0%D error.

2.1.3 AppendagesAppendages in this context refer to items

such as external shaft brackets, open propeller

shafts, bossings, the struts and pods of podded

propulsors, bilge keels, roll fins and rudders.

-

Appendages should be built to the full ex-ternal shape as designed.

- The manufacturing tolerances of append-ages should be within 0.2 mm.

- Surface finish should be at least as good asthat recommended for the hull model.

- Appendages should be located within 0.5mm of their design position.

2.2 Turbulence Stimulation2.2.1 Hull

The model should be fitted with a recog-

nised turbulence stimulator which should be

clearly described in the model documentation

and the report on the experiments. Suitable hull

turbulence stimulators include studs, wires and

sand grain strips. Figure 1, from Hughes and

Allan (1951) and NPL Report 10/59 (1960),

gives guidelines for the dimensions of studs

and the location of the studs as turbulence

stimulators on a raked stem of conventional

type.

Figure 1 Location of studs as turbulence stimulators


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Wires used for turbulence stimulation will

be typically between 0.5 mm and 1.0 mm di-

ameter, depending on position and model

speed, and be situated about 5% LPP aft of the

FP.

Sand strips used for turbulence stimulation

will typically comprise backing strips/adhesive

of 5 mm to 10 mm width covered with sharpedged sand with grain size around 0.50 mm,

with its leading edge situated about 5%LPP aft

of the FP.

A bulbous bow will additionally have tur-

bulence stimulators situated typically at of

the bulb length from its fore end.

2.2.2 AppendagesTurbulence stimulation should be applied to

appendages when laminar flow over the ap-

pendage is likely. Suitable turbulence stimula-

tors for appendages include studs, wires, sand

strips and Hama strips. Figure 2 provides de-

tails of the Hama strip, which will be located

near the leading edge of the appendage. Further

details of the Hama strip and a discussion of its

characteristics and applications, and the charac-

teristics of other stimulators, may be found in

ITTC (1990).

2.2.3 PropellerTurbulence stimulation will not normally be

applied to propellers used in self propulsion

and open water tests. There is however evi-

dence that turbulence stimulation may be nec-

essary on model propellers used in cavitation

tests.

Figure 2 Details of Hama strips as turbulence

stimulators

2.3 Preparation for Model Testing2.3.1 Ballasting and trimming (for resistance

and self propulsion tests)

The model should be loaded on displace-

ment and not on draught and should be run

within 0.2% of the correct calculated displace-

ment. The trim of the model should be such

that the errors in draught, if any, from the de-sign figure are the same at the forward and af-

ter perpendiculars. The model will normally be

tested without heel. The mean of the four

draughts, fore perpendicular, after perpendicu-

lar, port side amidships and starboard side

amidships, should be within 2.0 mm of the de-

signed figure. Hog or sag deformation should

not exceed acceptable tolerances, typically not

> 2.0 mm. For older models being re-tested, the

choice of an acceptable tolerance may depend

on resistance benchmark re-tests.


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2.3.2 Wax modelsBefore running, wax models should be left

fully sunk in the water, preferably for 36 hours

and not less than 12 hours. On re-floating, the

entire surface should be cleaned in the tank wa-

ter with a sponge or soft brush, particular care

being taken to remove all air bubbles and slime

from the surface. If the model has been in the

water for some weeks and has become en-

crusted with a crystalline deposit it should be

re-scraped and re-soaked, not merely sponged.

2.4 DocumentationThe particulars of the model(s) should be

collated in a report and/or included in a test re-

port, and should contain at least the following

information:

Hull Model:

Identification (model number or similar) Materials of construction Principal dimensions

Length between perpendiculars (LPP)

Length on waterline (LWL)

Breadth (B)

Draught (T)

Design displacement () (kg, fresh water)

Hydrostatics, including water plane areaand wetted surface area

Details of turbulence stimulation Details of appendages

Tolerances of manufacturePropeller Model:

Identification (model number or similar) Materials of construction Principal dimensions

Diameter

Pitch-Diameter Ratio (P/D)

Expanded blade Area Ratio (AE/AO)

Thickness Ratio (t/D)

Boss Diameter

Tolerances of manufacture

3 REFERENCES(1) Hughes, G. and Allan, J.F. (1951) Turbu-

lence Stimulation on Ship Models . Trans.

S.N.A.M.E., Vol.59, 1951.

(2) ITTC 1990, 19th International TowingTank Conference, Vol.1, pg.306.

(3) ITTC 1999, Model Manufacture, ShipModels, 22nd International Towing Tank

Conference, Seoul/Shanghai, Quality

Manual, 4.9-02-01-01, Revision 00.

(4) National Physical Laboratory, (1960),NPL Report SHR 10/59 Revised. Stan-

dard Procedure for Resistance and Pro-pulsion Experiments with Ship Models.

February, 1960.

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